There are a variety of established water and wastewater treatment systems. One type that has been in use for decades, in one form or another, is granular media gravity filtration. Granular media gravity filters, such as conventional deep bed sand filters, are used to strain out particles from a wastewater stream. Typically, wastewater is introduced into a sand filtration region from an influent chamber through inlet ports. The influent flows by gravity through the granular media filter, such as sand contained by a porous plate, to an effluent chamber. The granular media filter bed, which is typically divided into a series of adjacent cells, is then periodically cleaned using a variety of backwash procedures. See, for example, U.S. Pat. No. 4,152,265.
Another well known type of water and wastewater filtration is rotating cloth media filtration, often referred to as disk or drum filtration. In general, disk or drum filtration systems include a tank having an inlet and outlet and a rotatable filter frame positioned between the inlet and outlet. Cloth filter media is stretched over large drums or disk-type frame sections of the rotatable filter frame. During filtering, influent flows into the tank and passes through the cloth filter media, depositing the suspended solids on the filter media. The filtered effluent is then discharged from the tank outlet. See, for example, U.S. Pat. Nos. 4,090,965 and 4,639,315. The cloth filter media is periodically cleaned by a variety of procedures, including backwashing and/or high pressure sprays. See, for example, U.S. Pat. Nos. 5,374,360, 5,876,612 and 6,090,298. And, an improved self-aligning backwash system, among other things, for cleaning stationary cloth media is also described in U.S. Publication No. US2005/0161393A1, which is also incorporated herein.
In the early 1970's, a stationary cloth media filtration system was attempted in Europe. As shown schematically in FIG. 29, it is believed that this system included a filtration basin with a series of rectangular effluent chambers, each effluent chamber sandwiched between a pair of rectangular influent chambers. The vertical walls between influent chambers and the effluent chamber had a series of openings or windows across which cloth media screens were attached (FIG. 30). In operation, the wastewater to be treated was introduced into the influent chambers through subsurface gates. The influent was filtered by passing the flow through the screened windows of the filtration walls into the effluent chamber. From there, the filtered effluent flowed through subsurface gates to be discharged. The screens of this system were periodically cleaned by backwashing, which was accomplished by pulling a backwash header vertically up against the cloth filter media, using a submerged chain and sprocket arrangement.
This attempt at stationary cloth media filtration suffered from a number of problems. For example, if one of the filter screens failed, that whole section of the filtration system would have to be shut down, i.e., 2 influent chambers and associated effluent chamber. In addition, because of their design, the seals around the filtration screens were prone to leaks or failure, resulting in poor quality effluent. Similarly, since most of the moving parts, such as the chain and sprocket system needed for backwashing, were submerged, the chambers had to be dewatered before maintenance could be conducted. In short, this attempt at a cloth filtration system was very complicated and inefficient. It is believed that the system was a failure and was abandoned. As a result, the industry moved in the direction of rotating cloth media filtration methods, as generally described above.
Cloth media filtration systems require that the cloth filter media be subject to periodic cleaning, such as by backwashing and/or high pressure spray. Typical backwashing, in a system such as that shown in FIGS. 29 and 30, includes a suction header and backwash shoe assembly which is pressed directly against and pulled along the cloth filter media surface. In operation, a vacuum is applied to the suction header, pulling fluid through the cloth filter media and the backwash shoe in a direction opposite the flow direction during filtering (see FIG. 31). This reverse flow removes much of the accumulated solids caught in and blocking the cloth filter media. Typically, the suction header and shoe press directly against the cloth filter media (including the area where the cloth media is pulled against the frame assembly) in the conventional backwash arrangement (even when backwashing is not being conducted), which may put the cloth filter media under a preload. This may result in increased wear and premature break-through of the cloth filter media.
As indicated, regardless of whether cloth media filtration is stationary or rotating, it is necessary to periodically backwash the cloth filter media. In stationary cloth systems, backwashing is typically conducted in conjunction with traversing or traveling bridge type systems. In larger systems, a large number of backwash arms, sequencing valves and control wiers may be necessary to properly effectuate the backwashing operation. In addition, in known traversing backwash systems for stationary cloth media filtration, a number of sequencing valves are required to coordinate the backwashing operation to the desired arm and/or shoe. More complicated control wiring is also required.
Conventional rotating cloth media filtration also has some inherent limitations. For example, the filtration area is limited by the size of the disks/drums and/or tanks. Larger disks/drums require deeper and larger tanks, increasing their construction costs. The retrofit or construction of smaller tanks requires smaller disks, which reduces the filtration surface area.
Again, regardless of the type of filtration media used, uneven flow distribution over the volume of the filtration basin or region is a potential problem. For example, uneven or non-uniform flow distribution within the filtration basin or region often results in sludge settling, particularly in areas of low turbulence. This often results in the need for additional sludge removal equipment or increased system down time. In addition, non-uniform flow velocity across the filter may also result in increased sludge settling.
Thus, while the conventional deep bed sand and rotating disk filtration systems generally described above have been widely and successfully used in a variety of applications, each of these systems suffer from drawbacks inherent in or related to their size, design and/or application.